There exists a concern regarding the safety to users of wireless communications devices, such as mobile radios and cellular telephones, when these devices are operated in their transmit mode by reason that it is perceived the electromagnetic radiation associated with the transmission of the signals from such devices may pose a potential health hazard to the user. When using a hand-held wireless device the user holds the device near the ear and this positions the devices transmission antenna in close proximity to the user's hand and head as it transmits electromagnetic radiation and it is postulated that a portion of this radiation may be absorbed by the head and hand and that such absorption may cause health risks to the user. The perceived potential health hazard associated with wireless transmitting devices increases as the power of the signals transmitted from the device increases.
To address this perceived health risk many countries have adopted legislation requiring that the effective radiated power (ERP) of wireless devices be limited. For instance, the American National Standards Institute (ANSI) in association with the Institute of Electrical and Electronic Engineers, Inc. (IEEE) has adopted new standards for RF exposure viz. ANSI/IEEE C95.1-1992. The Federal Communications Commission (FCC) has adopted limits for maximum permissible exposure (MPE) time which, generally, are more restrictive than the previously adopted limits and guidelines and apply to land-mobile systems, such as cellular radio, pocket and hand-held radio telephones. These limits are based on recommended exposure guidelines published by the National Council on Radiation Protection and Measurements (NCRP) in “Biological Effects and Exposure Criteria for Radio frequency Electromagnetic Fields,” NCRP Report No. 86, Sections 17.4.1, 17.4.1.1, 17.4.2 and 17.4.3. Copyright NCRP, 1986, Bethesda, Md. 20814. The guidelines provide for exclusions to the regulations if it can be shown through laboratory procedures that exposure conditions do not exceed a certain specific absorption rate (SAR) or, alternatively, if the radiated power is below a certain level.
A quantification of MPE for uncontrolled environments is based on, inter alia, the electric and magnetic field strengths, the power density, the frequency range of the radio signal, the distance between the user's head and the transmitter radiating point. Thus, the MPE can be correlated to a maximum permitted transmission power, which may vary with the frequency range of a wireless device.
One method of meeting the applicable MPE limitations is to reduce the duty cycle of the transmitter. While techniques such as TDM (time division multiplexing) inherently activate the transmitter for less than 100% percentage of the total communication time, it is not an easy task to reduce the duty cycle of the transmitter on a source basis (i.e. without regard to network traffic) in such a way as to effectively control the duty cycle without negatively degrading the performance of the network and without averaging duty cycles over unreasonably long periods (tens of minutes or hours). Instead, since data is randomly transmitted over various time intervals, it is desired that the wireless device itself limit the effective transmit (active) time.
Some wireless devices are further capable of transmitting at several power levels, under command of a base station or independently, such as reducing output power to conserve battery power when close to a base station, or increasing output power to overcome interference.
In order to address the foregoing need for an effective method and system for managing the duty cycle of a transmitter the inventor developed the subject matter herein which makes effective use the quiet time inherent during a transmission.